The Human Nervous System¶
Part of 4.5 Homeostasis and Response.
The nervous system coordinates rapid responses to change. It is best understood as an information system that detects stimuli, processes them and sends impulses to effectors.
Learning Objectives¶
| ID | Official specification wording | Main teaching sections |
|---|---|---|
4.5.2-lo-1 |
4.5.2.1 Students should be able to explain how the structure of the nervous system is adapted to its functions. 4.5.2.1 The nervous system enables humans to react to their surroundings and to coordinate their behaviour. 4.5.2.1 Information from receptors passes along cells (neurones) as electrical impulses to the central nervous system (CNS). The CNS is the brain and spinal cord. The CNS coordinates the response of effectors which may be muscles contracting or glands secreting hormones. |
Overview, Neurones |
4.5.2-lo-2 |
4.5.2.1 stimulus receptor coordinator effector response Students should be able to explain how the various structures in a reflex arc – including the sensory neurone, synapse, relay neurone and motor neurone – relate to their function. Students should understand why reflex actions are important. 4.5.2.1 Reflex actions are automatic and rapid; they do not involve the conscious part of the brain. 4.5.2.1 Students should be able to extract and interpret data from graphs, charts and tables, about the functioning of the nervous system. 4.5.2.1 Students should be able to translate information about reaction times between numerical and graphical forms. |
Neurones, Synapses, The Reflex Arc |
4.5.2-lo-3 |
4.5.2.2 The brain controls complex behaviour. It is made of billions of interconnected neurones and has different regions that carry out different functions. 4.5.2.2 Students should be able to identify the cerebral cortex, cerebellum and medulla on a diagram of the brain, and describe their functions. 4.5.2.2 (HT only) Students should be able to explain some of the difficulties of investigating brain function and treating brain damage and disease. 4.5.2.2 (HT only) Neuroscientists have been able to map the regions of the brain to particular functions by studying patients with brain damage, electrically stimulating different parts of the brain and using MRI scanning techniques. The complexity and delicacy of the brain makes investigating and treating brain disorders very difficult. |
The Brain |
4.5.2-lo-4 |
4.5.2.3 Students should be able to relate the structures of the eye to their functions. This includes: 4.5.2.3 • accommodation to focus on near or distant objects 4.5.2.3 • adaptation to dim light. 4.5.2.3 The eye is a sense organ containing receptors sensitive to light intensity and colour. 4.5.2.3 Students should be able to identify the following structures on a diagram of the eye and explain how their structure is related to their function: 4.5.2.3 • retina 4.5.2.3 • optic nerve 4.5.2.3 • sclera 4.5.2.3 • cornea 4.5.2.3 • iris 4.5.2.3 • ciliary muscles 4.5.2.3 • suspensory ligaments. 4.5.2.3 Accommodation is the process of changing the shape of the lens to focus on near or distant objects. 4.5.2.3 To focus on a near object: 4.5.2.3 • the ciliary muscles contract 4.5.2.3 • the suspensory ligaments loosen 4.5.2.3 • the lens is then thicker and refracts light rays strongly. 4.5.2.3 To focus on a distant object: 4.5.2.3 • the ciliary muscles relax 4.5.2.3 • the suspensory ligaments are pulled tight 4.5.2.3 • the lens is then pulled thin and only slightly refracts light rays. 4.5.2.3 Two common defects of the eyes are myopia (short sightedness) and hyperopia (long sightedness) in which rays of light do not focus on the retina. 4.5.2.3 • Generally these defects are treated with spectacle lenses which refract the light rays so that they do focus on the retina. 4.5.2.3 • New technologies now include hard and soft contact lenses, laser surgery to change the shape of the cornea and a replacement lens the eye. 4.5.2.3 Students should be able to interpret ray diagrams, showing these two common defects of the eye and demonstrate how spectacle lenses correct them. |
The Eye |
Overview¶
The nervous system allows humans to react to their surroundings and coordinate behaviour. It works alongside the hormonal (endocrine) system:
- Nervous responses — fast (milliseconds), short-lived, electrical in nature.
- Hormonal responses — slower (seconds to minutes), longer-lasting, chemical in nature.
The nervous system is divided into: - Central nervous system (CNS) — brain and spinal cord. - Peripheral nervous system (PNS) — all nerves that run to and from the CNS, including sensory and motor nerves.
Actions can be voluntary (requiring conscious decision, e.g. picking up a cup) or involuntary/reflex (automatic and unconscious, e.g. withdrawing from pain).
Neurones¶
Electrical signals called nerve impulses travel along specialised cells called neurones (or nerve cells). A nerve is a bundle of many neurones.
Types of Neurone¶
| Type | Direction of travel | Location |
|---|---|---|
| Sensory neurone | Receptor → CNS | PNS |
| Relay neurone | Within the CNS | CNS (spinal cord/brain) |
| Motor neurone | CNS → effector | PNS |
Structure of a Neurone¶
- Axon — long projection that carries the impulse.
- Myelin sheath — insulating fat layer around the axon; speeds up impulse transmission and prevents electrical short circuits.
- Cell body — contains the nucleus; located at one end of the neurone.
- Dendrons and dendrites — shorter extensions from the cell body that receive signals from other neurones.
Sensory Receptors¶
Receptors are groups of specialised cells that detect a specific stimulus and convert it into an electrical signal. Different receptors respond to different stimuli: temperature, pressure, light, chemicals, etc.
Synapses¶
A synapse is the junction between two neurones (or between a neurone and an effector). Impulses cannot jump the gap directly — they are transmitted chemically:
- The electrical impulse reaches the end of the first axon (pre-synaptic membrane).
- The impulse triggers the release of neurotransmitters from vesicles into the synaptic cleft (the gap).
- Neurotransmitters diffuse across the cleft and bind to complementary receptors on the second neurone (post-synaptic membrane).
- Binding triggers a new electrical impulse in the second neurone.
Receptors are only on one side of the synapse, so impulses can only travel in one direction. Some drugs (e.g. heroin) act by interfering with neurotransmitter receptors at synapses.
The Reflex Arc¶
A reflex is an involuntary, automatic, rapid response to a stimulus. Reflexes do not involve the conscious brain — they run through the spinal cord only. This makes them faster and ensures they happen the same way every time.
Pathway (in order):
- Stimulus detected by receptor.
- Impulse travels along sensory neurone to the spinal cord (CNS).
- Impulse passes across a synapse to a relay neurone in the spinal cord.
- Impulse travels along motor neurone from spinal cord to effector.
- Effector (muscle or gland) produces the protective response.
Examples: withdrawing a hand from a hot surface; the knee-jerk reflex.
Investigating Reaction Time¶
Reaction time can be measured using the ruler-drop test. The time taken for a person to catch a falling ruler indicates how fast nerve impulses travel through their system. Variables such as caffeine intake or distractions can be tested.
The Brain¶
The brain is the main coordination centre of the CNS. It contains billions of interconnected neurones organised into functional regions.
| Region | Functions |
|---|---|
| Cerebral cortex | Conscious thought, intelligence, personality, memory, language, sensory processing |
| Cerebellum | Coordination of movement, balance, muscle control |
| Medulla oblongata | Unconscious automatic control: heart rate, breathing rate |
| Hypothalamus | Homeostasis (temperature, water balance); produces hormones controlling the pituitary gland |
| Pituitary gland | Releases hormones (e.g. ADH, FSH, LH) |
Investigating the Brain¶
Scientists map brain regions to functions using several methods:
- MRI scans — magnetic fields and electromagnetic waves show brain structure and activity. No ionising radiation; safer for repeated use.
- CT scans — X-rays produce structural images; useful for identifying brain damage (e.g. stroke damage).
- PET scans — radioactive tracers highlight areas of high metabolic activity; can identify regions more or less active than normal.
- Electrical stimulation — applying small electrical currents to specific regions and recording responses; EEGs monitor brain electrical activity.
- Studying brain damage — patients with lesions in specific brain regions show loss of specific functions, revealing what those regions do.
Studying the brain is difficult because it is incredibly complex and accessing it involves significant surgical risk.
The Eye¶
The eye is a sensory organ containing receptors that detect light. It converts light signals into electrical impulses sent to the brain via the optic nerve.
Structure and Functions¶
| Structure | Function |
|---|---|
| Cornea | Transparent outer layer; refracts (bends) light entering the eye |
| Iris | Coloured ring of muscle; controls pupil size |
| Pupil | Hole in the iris through which light enters |
| Lens | Focuses light precisely onto the retina; changes shape for accommodation |
| Ciliary muscles | Contract or relax to change lens shape |
| Suspensory ligaments | Hold the lens in place; go slack when ciliary muscles contract |
| Retina | Contains rod and cone cells; converts light to electrical signals |
| Optic nerve | Carries impulses from retina to brain |
Rods and Cones¶
| Rod cells | Cone cells | |
|---|---|---|
| Location in retina | Mainly periphery | Mainly fovea |
| Colour vision | No (black and white only) | Yes (3 types: red, green, blue) |
| Sensitivity | Very high — works in dim light | Lower — works in bright light |
The Pupil Reflex¶
The iris contains two sets of antagonistic muscles:
- In bright light: circular muscles contract, radial muscles relax → pupil constricts (gets smaller) → less light enters → protects retina.
- In dim light: radial muscles contract, circular muscles relax → pupil dilates (gets larger) → more light enters → improves vision.
Accommodation (Focusing)¶
The lens changes shape to focus on objects at different distances:
Focusing on near objects: 1. Ciliary muscles contract. 2. Suspensory ligaments go slack. 3. Lens becomes fatter and more curved → greater refraction → light focuses on retina.
Focusing on distant objects: 1. Ciliary muscles relax. 2. Suspensory ligaments become taut. 3. Lens becomes thinner and flatter → less refraction → light focuses on retina.
Eye Defects and Corrections¶
Myopia (short-sightedness): - Cannot see distant objects clearly; focal point falls in front of the retina. - Cause: eyeball too long or lens too curved. - Correction: concave (diverging) lens — spreads light rays before they reach the eye.
Hyperopia (long-sightedness): - Cannot see near objects clearly; focal point would fall behind the retina. - Cause: eyeball too short or lens too flat. - Correction: convex (converging) lens — brings light rays closer together before they reach the eye.
Treatments: - Glasses or contact lenses (hard or soft). - Laser surgery — reshapes the cornea permanently (mainly for myopia; some risk of infection or visual impairment). - Lens replacement — artificial lens implanted surgically.
Cataracts: cloudy patches form on the lens, causing blurred vision. Treated by replacing the lens with an artificial one.
Colour blindness: faulty cone cells (usually red or green cones) prevent normal colour discrimination. Currently no cure.
Common Confusions¶
- Reflex arc vs conscious response: reflexes do not involve the brain (they go through the spinal cord); conscious responses involve the cerebral cortex. A reflex is faster because the pathway is shorter.
- Relay neurone placement: relay neurones are in the CNS (spinal cord), not in the periphery.
- Accommodation muscle logic: ciliary muscles contracting makes the lens fatter (for near vision). Many students assume contracting pulls the lens tighter — but it is the suspensory ligaments that hold the lens taut, and when they go slack (because ciliary muscles contract inward), the elastic lens bulges.
- Pupil vs iris: the pupil is the gap; the iris is the muscle ring around it. The iris muscles change the pupil size, not vice versa.
- Myopia correction: myopia needs a concave lens (not convex). Long-sightedness needs convex.
Key Terms¶
- Reflex arc: the pathway taken by impulses in a rapid automatic response.
- Sensory neurone: a neurone that carries impulses from receptors to the central nervous system.
- Relay neurone: a neurone within the CNS that connects sensory and motor neurones.
- Motor neurone: a neurone that carries impulses from the central nervous system to effectors.
- Synapse: the junction between two neurones; impulses cross by neurotransmitter diffusion.
- Neurotransmitter: a chemical released at a synapse that carries the signal across the synaptic cleft.
- Accommodation: the process by which the lens changes shape to focus on near or distant objects.
- Vasodilation: widening of blood vessels near the skin surface to increase heat loss.
- Vasoconstriction: narrowing of blood vessels near the skin surface to reduce heat loss.
- Myelin sheath: the insulating fatty layer around a neurone's axon that speeds impulse transmission.
- Retina: the layer at the back of the eye containing rod and cone cells that detect light.
- Fovea: the region of the retina with the highest concentration of cone cells; gives the sharpest colour vision.
- Ciliary muscles: muscles in the eye that change the shape of the lens for accommodation.
- Suspensory ligaments: fibres that hold the lens in position in the eye.
- Myopia: short-sightedness; corrected by a concave lens.
- Hyperopia: long-sightedness; corrected by a convex lens.
- Cerebral cortex: the region of the brain responsible for conscious thought, intelligence, memory and language.
- Cerebellum: the region of the brain that coordinates movement and balance.
- Medulla oblongata: the region of the brain that controls unconscious processes such as heart rate and breathing.